Alot of people don't grow perfectly aligned teeth when they are growing up, there may be some gaps in between the teeth or some crooked teeth, these esthetic problems can be fixed by crowning or orthodontics treatments.
Coloration on teeth mostly happens when people don't take good care of their oral health. The most common thing that leads people to search for esthetic dentistry is this.
Another reason is traumas to the mouth. If this is the reason, it is very common that people need to improve their esthetics because of broken teeth etc.
One of the other reasons people look for esthetic dentistry is the recession of gums which happens because of lack of oral hygiene. If this is the case a good periodontologist should evaluate and fix the problem.
Our teeth should be in line and compatible in color with our face. A perfect esthetic plan always takes into account peoples face shape, skin color, eye color, hair color because the teeth must be compatible with the general look of a persons face. For example if the patients jaw is narrow then the teeth should be a little narrower than usual to achieve compatibility.
When people think of esthetic dentistry the first two branches that usualy come to mind are Prosthodontics and Orthodontics but all the branches of dentistry are elements in esthetic dentistry. It must be thought of as a whole since every branch may come into play in esthetic solutions according to the patients needs.
Everyone has the right to want the perfect looking teeth for them but to achieve this result time may vary alot. For example if the only problem is a simple coloration of the teeth, a dental bleaching can fix this problem in around 45 minutes but if there is also crooks and gaps in the teeth then you will want orthodontics or prosthesis' to fix the look. If your gum looks swollen or discolored you will need a periodontal treatment to make them pinkish in color and healthy again.
For the prosthetic solutions such as crowns, bridges and dentures you may require root canal treatment before the prosthodontic applications because of the infected root of the tooth and at this stage your prosthodontist may want your endodontist to apply fiber posts to the tooth canal to increase the crown's durability.
About the materials used in crowns, bridges and dentures; it all varies from brand to brand and the expertise of the dentist about the materials, but commonly Zirconia or Full Porcelain Crowns are the more esthetic looking solutions when compared to Metal Based Porcelain Crowns since any metal inside the crown will create a dark layer inside the crown which makes it hard to look transparent and natural like your natural teeth.
In short, every branch of dentistry should be used for the best esthetic outcome.